PROJECT SUMMARY After years of assuming that neurological diseases are caused by direct damage to neurons, we now know that impaired astrocyte physiology and function precedes and is essential for the progression of many of these diseases. This revelation hints toward the reason why anti-epileptic drugs that exclusively target neurons do not prevent the development of epilepsy after traumatic brain injury (TBI), the largest group of acquired epilepsies. For more than a decade, data have accumulated showing that astrocytes become reactive and lose their homeostatic functions indispensable for normal neuronal operation in epilepsy patients and animal models. Identifying and targeting astrocytic molecular mechanisms contributing to this dysfunction might provide the long-missing puzzle piece to interrupt the slow progression from TBI to post-traumatic epilepsy (PTE). Yet, astrocytic molecular mechanisms driving PTE have not been identified. This may be due to the complexity of TBI, which induces many pathobiological mechanisms in parallel, as well as the lack of unbiased approaches to identifying new targets. To address these problems, a new mouse model of post-traumatic epilepsy that recapitulates diffuse/concussive TBI without focal injury was developed reducing the number of pathobiological mechanisms that have to be taken into consideration. This model was used to identify specific molecular targets that might contribute to astrocyte dysfunction after diffuse TBI using unbiased RNA sequencing of the astrocytic transcriptome. This approach pointed to the downregulation of cell communication pathways, which was corroborated by the reduced spread of stimulated astrocytic calcium waves in vivo after diffuse TBI. Astrocytes are coupled by and communicate via gap junctions of which Cx43 is a major component. Importantly, altered astrocyte gap junction coupling has been reported in patients who suffer from temporal lobe epilepsy and in animal models of this disease. In addition to forming gap junctions, Cx43 is present in undocked hemichannels. These abnormally open under pathological conditions, releasing neurotoxic molecules such as ATP and glutamate. Our preliminary results confirm abnormal expression pattern of Cx43, a loss of gap junction plaque formation and relocalization in the cytoplasm following TBI. Cx43 localization and activity is regulated by several sites within the Cx43 carboxy-terminus (CT) that mediate protein?protein interaction and allow for post-translational modifications. This proposal uses two proprietary Cx43 mimetic peptides, aCT1 and JM2, which modulate Cx43 localization and function by increasing Cx43 gap junction plaque formation and blocking Cx43 hemichannel activity, respectively. The overall objective of this proposal is to determine in which way abnormal Cx43 following diffuse TBI affects astrocyte function and to develop new therapeutic strategies to prevent PTE using Cx43 mimetic peptides.